There are many known applications for visual marking. Visual marking may be used to mark the position of a site or an area of interest, the position of an individual or a group of individuals or the location of some unit(s) or equipment. For example, in the battlefield, individuals and units sometimes need to mark their position in order to allow other individuals or units to observe and detect their position. In another example, airport operators need to mark the location and outline of runways in order to enable safer landings and takeoffs.
In many cases, it may be advantageous to use covert marking to mark one's position (be it an individual(s), a unit(s) or a site). Covert marking may prevent detection by an unfriendly or unauthorized observer. Effectiveness and covertness of the marking equipment should preferably be maintained day and night. An example of one application of covert marking is disclosed in U.S. Pat. No. 7,023,361 to Wallace, et al. which relates to a covert runway lighting apparatus and method.
It is known to use Near Infrared (NIR) light sources for nighttime self marking. NIR light sources are invisible to the naked eye, but can be detected, during nighttime, by using night vision goggles (NVG), CCD/CMOS/Vidicon imaging detectors and similar equipment. Such NIR light sources may include lamps, LEDs and semiconductor lasers. An example of a NIR beacon is disclosed in U.S. Pat. No. 4,912,334 to Anderson.
Imaging systems employing the thermal portion of the infrared spectrum (e,g, Mid-Wave Infrared (MWIR) and/or Long-Wave Infrared (LWIR)), are in use in military applications as well as in commercial applications. These systems generate a picture of the infrared radiance differences between objects within their field of view. Imaging systems which operate at the thermal portion of the infrared spectrum (MWIR and/or LWIR) are commonly referred to as “Forward Looking Infrared” or “FUR”.
A self-marking device which is visible to FUR has been proposed by Thermal Beacon, Tactronics, Ion-Optics, CI-Systems and other companies. These are based on blackbody radiance of source-objects in the 3-5 μm or the 8-14 μm wavelength bands, where blackbody emission is significant and the atmosphere is transparent.
The source of thermal radiance (the term “thermal radiance” will refer to radiation in the 3-5 μm and/or the 8-14 μm wavelength bands) is usually heaters which achieve a temperature of a few hundreds of Celsius degrees. This is because the blackbody radiation peaks within 3-5 μm in these wavelengths (and thus the energy conversion to the required wavelength band is high), and because there are materials that can be heated to this temperature without melting, oxidizing or disintegrating even in the presence of air. These heaters may be thin carbon films, cavity black-body or other kinds of resistors. These beacons typically emit into a wide solid angle, because of the size of the “black-body” object. In some cases, they include a window which blocks the visible and sometimes NIR energy, so that the beacon is covert to NVG, CCD/CMOS/Vidicon based cameras, and the naked eye.
U.S. Pat. No. 6,777,701 to Inbar discloses a system which utilizes high temperature low mass filament as the emitter, mainly due to the fast response to pulses. Similar emitters are also described in U.S. Pat. No. 5,939,726 to Wood. The temperatures achieved by commercially available filament emitters, such as those available from Cal-Sensors, Assignees of U.S. Pat. No. 5,939,726 is 1000K for the Pulsable Emitters, and 1170K for the steady-state emitters. The filament emitters available from for Ion Optics and SciTek and others achieve similar results.
U.S. Pat. No. 5,438,233 to Boland, et al. discloses a lamp configuration particularly for infrared radiation and that provides an internal arrangement and environment adapted to overcome failure modes of previous broadband infrared sources. The disclosed lamp configuration may incorporate optical elements including spectral filters and lenses enabling wavelength selection, beam shaping, external focusing, collimating, and wave front shaping. It also facilitates optical coupling to external devices including a rotating mirror, shutter and modulator devices. Boland et al. propose to use an inert gas mixture environment within the lamp to prevent oxidization, and therefore allows the filament to reach higher temperatures without failure due to chemical reactions with the surrounding gas.
U.S. Pat. No. 6,087,775 to Levinson, et al. discloses a lamp assembly which comprises an incandescent lamp, and in particular a Halogen lamp, capable of generating light. The incandescent lamp comprises an incandescent lamp tube and at least one filament. The assembly also comprises a shroud separate from the incandescent lamp and mounted in communication with the lamp tube on an exterior of the incandescent lamp tube. The shroud also comprises a coating disposed on the reflecting section of the shroud for reflecting energy having predetermined wavelengths emitted by the incandescent lamp, an in particular for reflecting energy whose wavelength is within the infrared band.
U.S. Pat. No. 6,567,248 to Schmidt, et al. is an example of a Tri-Spectrum Landing Light, which in addition to being capable to provide FLIR emission, is capable of providing visible and infrared light. Schmidt, et al. proposes to use three different modalities to enable the tri-spectrum emission capability.